This invention relates to a novel polyimide film. More particularly, it relates to a novel polyimide film which has excellent characteristics (a sufficiently high elastic modulus, a low water absorption, a small coefficient of moisture-absorption expansion, a small coefficient of linear expansion and a high dimensional stability, etc.) and is adequately usable as a base film for various electric/electronic equipments such as an adhesive film for flexible print connection boards or in packaging semiconductors, a magnetic recording film, a hard-disk suspension connection base, etc.
In general, polyimide films are excellent in various properties (heat resistance, low temperature properties, chemical resistance, electrical characteristics, etc.) among various organic polymers. Therefore, polyimide films have been widely employed as materials of electric/electronic equipments as well as in the fields of cosmology/aeronautics and electronic communication. In particular, it is required in recent years that these polyimide films are not only excellent in heat resistance but have various functions so as to meet the demands.
For example, it is desired that a polyimide film to be used as a base film for flexible print connection boards has a high elastic modulus, a small coefficient of moisture-absorption expansion and a small coefficient of linear expansion. When a polyimide film having a large coefficient of moisture-absorption expansion and a large coefficient of linear expansion is employed, the obtained flexible print connection board undergoes curling or warpage. In particular, a board base film having a high dimensional stability should be used in a flexible print connection board for plasma display (PDP), since it is to be used in a large area compared with those for other uses.
As the polyimide to be used in the electric/electronic equipments as described above, it has been a practice to use a polyimide obtained by the polycondensation of pyromellitic acid dianhydride with 4,4xe2x80x2-diaminodiphenhyl ether, since it is excellent in heat resistance and electrical insulating properties and, therefore, usable in these equipments to be used at high temperature. By taking advantage of the high dimensional stability, films made of this polyimide are also used in flexible print connection boards, etc. In recent years, however, it becomes necessary that base films to be used in IC, LSI, etc. have a further improved proccessability and an elevated accuracy. In its turn, the polyimide serving as the base film material should have an elevated elastic modulus, a lowered coefficient of linear expansion, a reduced water absorption, etc. Thus, various attempts have been made to satisfy these requirements. For example, it is reported to elevate the elastic modulus by using p-phenylenediamine as a diamine component thereby giving a three-component polyimide composed of pyromellitic acid dianhydride, 4,4xe2x80x2-diaminodiphenyl ether and p-phenylenediamine in JP-A-60-210629, JP-A-64-16832, JP-A-64-16833, JP-A-64-16834, JP-A-1-131241 and JP-A-1-131242 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined Japanese patent publicationxe2x80x9d). Moreover, it is also attempted to add 3,3xe2x80x2,4,4xe2x80x2-bipehnyltetracarboxylic acid dianhydride to the above-described three components to thereby give a four-component polyimide having a further elevated elastic modulus. For example, JP-A-59-164328 and JP-A-61-111359 discloses such a four-component polyimide. Furthermore, an attempt to improve the physical properties of the four-component polyimide by adding the monomers in a controlled order in the step of the polymerization is reported in, for example, JP-A-5-25273. Also, it is reported to use an acid having a structure similar to p-phenylenebis(trimellitic acid monoesteracid anhydride) in JP-A-63-189490 (examined Japanese Patent Publication No. 7-88495), JP-A-3-60182 (Japanese Patent No. 2,712,597), JP-A-9-77871 and JP-A-10-36506.
As described above, more and more demands are imposed upon polyimide films which are employed in electric/electronic equipments and, therefore, various studies have been made to satisfy these requirements. However, there has been proposed no polyimide film so far having sufficiently excellent characteristics (a sufficiently high elastic modulus, a low water absorption, a small coefficient of moisture-absorption expansion, a small coefficient of linear expansion and a high dimensional stability).
Under these circumstances, one object of the present invention is to provide a polyimide film having sufficiently excellent characteristics (a sufficiently high elastic modulus, a low water absorption, a small coefficient of moisture-absorption expansion, a small coefficient of linear expansion and a high dimensional stability). Another object of the present invention is to provide various electric/electronic equipment bases with the use of the polyimide film.
To achieve the above-described objects, the present invention provides a polyimide film, a laminate for flexible print connection boards, an adhesive film, a base film for magnetic recording and a hard disk suspension connection base each will be specified below.
(1) A polyimide film having a tensile elastic modulus of 700 kg/mm2 or less and a coefficient of moisture-absorption expansion of 20 ppm or less.
(2) The polyimide film as described in the above (1) having a coefficient of linear expansion at 100 to 200xc2x0 C. of 5 to 15 ppm.
(3) The polyimide film as described in the above (1) or (2) having a water absorption of 3.0% or less.
(4) The polyimide film as described in the above (3) having a water absorption of 2.0% or less.
(5) The polyimide film as described in any of the above (1) to (4) which contains a repeating unit represented by the following general formula (1) in its molecule: 
wherein R1 represents a divalent organic group selected from among: 
(wherein R4 represents CH3xe2x80x94, Clxe2x80x94, Brxe2x80x94, Fxe2x80x94 or CH3Oxe2x80x94); and R represents a divalent organic group represented by the following general formula: 
(wherein n is an integer of 1 to 3; and X represents a monovalent substituent selected from among a hydrogen atom, a halogen atom, a carboxyl group, a lower alkyl group having 6 or less carbon atoms and a lower alkoxy group having 6 or less carbon atoms), or the following general formula: 
(wherein Y and Z may be the same or different and each represents a monovalent substituent selected from among a hydrogen atom, a halogen atom, a carboxyl group, a lower alkyl group having 6 or less carbon atoms and a lower alkoxy group having 6 or less carbon atoms; and A represents a divalent linking group selected from among xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94SO2xe2x80x94 and xe2x80x94CH2xe2x80x94.)
(6) The polyimide film as described in the above (5) which further contains a repeating unit represented by the following general formula (2) in its molecule in addition to said repeating unit represented by the general formula (1): 
wherein R is as defined in the general formula (1); and R3 represents a tetravalent organic group selected from among: 
(7) The polyimide film as described in the above (5) or (6) which further contains a repeating unit represented by the following general formula (3) in its molecule: 
wherein R is as defined in the general formula (1).
(8) The polyimide film as described in the above (5) or (6) which contains a repeating unit represented by the following general formula (4) and another repeating unit represented by the following general formula (5) in its molecule: 
wherein R2 represents a divalent organic group selected from: 
(wherein R2 is as defined in the general formula (4); and R3 is as defined in the general formula (2)).
(9) The polyimide film as described in the above (5) or (6) wherein the main repeating units contained in its molecule are those represented by the following general formulae (6) to (9): 
(10) The polyimide film as described in the above (9) which satisfies the requirement that (a+b)/s, (a+c)/s, (b+d)/s and (c+d) Is each falls within the range of 0. 25 to 0. 75, wherein a, b, c and d represent respectively the numbers of the repeating units represented by the above general formulae (6) to (9) and s represents a+b+c+d.
(11) A laminate for flexible print connection boards which is obtained by forming a metal layer at least on one face of the polyimide film as described in any of the above (1) to (10).
(12) The laminate for flexible print connection boards as described in the above (11), wherein said metal layer is laminated via a thermosetting adhesive.
(13) The laminate for flexible print connection boards as described in the above (11), wherein said metal layer is laminated via a thermoplastic polyimide adhesive.
(14) The laminate for flexible print connection boards as described in any of the above (11) to (13), wherein at least one face of said polyimide film is subjected to at least one treatment selected from among heat treatment, corona treatment, plasma treatment and coupling agent-treatment.
(15) An adhesive film obtained by forming an adhesive layer on at least one face of the polyimide film as described in any of the above (1) to (10).
(16) The adhesive film as described in the above (15) wherein said adhesive layer comprises a thermosetting adhesive.
(17) The adhesive film as described in the above (15) wherein said adhesive layer comprises a thermoplastic polyimide adhesive.
(18) The polyimide film as described in any of the above (1) to (10) to be used as a base film for magnetic recording.
(19) The polyimide film as described in any of the above (1) to (10) to be used as a hard disk suspension connection base.